Adriano Zecchina

Role of Exposed Metal Sites in Hydrogen Storage in MOFs

The role of exposed metal sites in increasing the H2 storage performances in metal-organic frameworks (MOFs) has been investigated by means of IR spectrometry. Three MOFs have been considered: MOF-5, with unexposed metal sites, and HKUST-1 and CPO-27-Ni, with exposed Cu(2+) and Ni(2+), respectively. The onset temperature of spectroscopic features associated with adsorbed H2 correlates with the adsorption enthalpy obtained by the VTIR method and with the shift experienced by the H-H stretching frequency. This relationship can be ascribed to the different nature and accessibility of the metal sites. On the basis of a pure energetic evaluation, it was observed that the best performance was shown by CPO-27-Ni that exhibits also an initial adsorption enthalpy of -13.5 kJ mol(-1), the highest yet observed for a MOF. Unfortunately, upon comparison of the hydrogen amounts stored at high pressure, the hydrogen capacities in these conditions are mostly dependent on the surface area and total pore volume of the material. This means that if control of MOF surface area can benefit the total stored amounts, only the presence of a great number of strong adsorption sites can make the (P, T) storage conditions more economically favorable. These observations lead to the prediction that efficient H2 storage by physisorption can be obtained by increasing the surface density of strong adsorption sites.

Well defined CuI(NO), CuI(NO)2 and CuII(NO)X (X = O− and/or NO 2 − ) complexes in CuI-ZSMS prepared by interaction of H-ZSM5 with gaseous CuCl

In this note an exchange procedure of the acidic protons of H-ZSM5 by CuI ions through reaction with CuCl in the gas phase is described. In the so obtained CuI-ZSM5 exchanged zeolite the CuI ions are in well defined configuration and form with NO mono and di-nitrosyl complexes of high structural and spectroscopic quality. The CuI(NO)2 species are transformed at RT into CuII(NO)X (X=O− and/or NO2−) species which could represent an intermediate in NO decomposition.

Surface acidity and basicity: General concepts

Abstract A very general definition of acids and basis, following both Bronsted and Lewis theories, is the starting point to introduce the concept of the acid and basic strength of surface sites. The surface of an oxide is described as a bidimensional organization of acid–base pair (AB), whose strength can be measured by probe molecules. It is shown that in order to obtain a reasonable scale of surface acidity and basicity, probe molecules characterized by minimal interaction energy must be used. Among the probe molecules, which can be employed to measure surface acidity, carbon monoxide is the most useful. On the contrary, a probe molecule specific and highly sensitive for basic sites is missing. As case studies, Lewis and Bronsted sites on ZnO and Bronsted acid groups of zeolites are discussed.

IR investigation of polymerization centres of the phillips catalyst

The CO-reduced Phillips catalyst (Cr(II) ions supported on silica) has been studied by IR spectroscopy. The following points have been examined: (i) CO/C2H4 interaction; (ii) vibrational features of the polymer; (iii) its reactivity towards oxygen; (iv) the effect of CO on the spectrum of the polymer and vice versa. The number of active sites is shown to increase during the polymerization and to reach some 10% of the total loading. The spectral features of the Cr ions involved in the active sites are consequently visible in IR: before polymerization they chemisorb CO more strongly and form stable CO-ethylene complexes; after polymerization they can still adsorb CO in a distinguishable form (stretching frequency at 2173 cm−1). The polymer chain does not show any terminal group such as methyl or vinyl: a cyclic structure is proposed. The chain may exist in either alkyl or alkylidenic form, in equilibrium with each other and differing by a proton reversibly transferred to a nearby oxygen. A model for the polymerization mechanism is advanced involving a metallacyclobutane intermediate, and accounting for the observed absence of scrambling among H atoms.

Probing the acid sites in confined spaces of microporous materials by vibrational spectroscopy

The use of IR spectroscopy for the evaluation of the Lewis and Bronsted acidity of microporous systems is illustrated having recourse to examples concerning zeolites, heteropolyacids and sulfonated membranes (NAFION). Methods based on the perturbation of the OH modes by interaction with basic probe molecules are illustrated as well as on the perturbation of the internal modes of the probe itself. The use of H2 as probe is also debated by discussing new data specifically obtained for this review. The illustrated case examples are mainly obtained from the experience gained by the Turin group in 20 years.

Comparative IR-spectroscopic study of low-temperature H2 and CO adsorption on Na zeolites

Extraframework cation sites in the sodium forms of the zeolites ZSM-5, mordenite, Linde-4A and faujasite-type X and Y have been investigated by using low-temperature adsorption of dihydrogen and carbon monoxide as IR spectroscopic probes. The extent of H—H and C—O bond polarization was found to be dependent not only on the cation electrostatic field, but also on the neighbouring oxygen atoms of the zeolite framework. The influence of these oxygen atoms is most keenly felt by adsorbed molecular hydrogen, but they also affect the IR frequency shift of the stretching vibration of adsorbed carbon monoxide. The Si : Al ratio of the zeolite framework modulates the basic strength of the oxygen atoms, and this was found to be reflected in the IR stretching frequency of both adsorbed molecules, H2 and CO.

IR studies of CO and NO adsorbed on well characterized oxide single microcrystals

Abstract A systematic investigation of the surface morphology and of the vibrational properties of CO and NO adsorbed on simple oxides microcrystals (like MgO, NiO, NiO-MgO, CoO-MgO, ZnO, ZnO-CoO, α-Cr 2 O 3 , α-Al 2 O 3 , MgAl 2 O 4 and other spinels, TiO 2 , ZrO 2 and other oxides of a similar structure) with regular crystalline habit and exposing thermodynamically stable and neutral faces, is presented with the aim to elucidate the spectroscopic manifestations of CO and NO adsorbed on well defined crystallographic positions. In particular the structure of CO and NO adsorbed on the cationic sites of extended faces of these model solids is presented and discussed with the aim of elucidating the nature of the Me x+ ··· CO/NO bond (Me x+ = non transition metal ion or transition metal ion). When non transition metal ions are involved, the molecule-cation interaction is predominantly electrostatic. This leads to an increase of the CO stretching frequency, which is roughly proportional to the polarizing field. On the contrary, when transition metal ions are involved, beside the predominant electrostatic interactions, a small contribution to the bond stability comes also from d-π overlap forces, which, although not very important from the energetic point of view, greatly influence the static and dynamic dipoles localized on the adsorbed molecules. Consequently, the strength of the dipole-dipole interactions occurring in the ordered adlayers of CO and NO adsorbed on transition and non transition metal oxide surfaces are resulted remarkably different. On these well defined surfaces, the effects influencing the half-width (FWHM) of the CO and NO stretching peaks have also been considered. It has been calculated that the FWHM is a very sensitive parameter of the surface perfection. In a few cases (ZnO, α-Cr 2 O 3 , etc.) FWHM values comprised in the 1.5–3.7 range have been obtained, which are indicative of a single-crystal quality of the exposed faces. These spectroscopic results were compared with those obtained with quantum calculations. Finally the activity towards CO and NO of perfect, low index faces and of more defective situations (like those associated with edges, steps and corners) are compared, in order to have a better insight on the role of surface defectivity in catalytic reactions.

Two Coordination Modes of CO in Zeolites: A Temperature-Dependent Equilibrium

CO interacts with exchangeable cations M+ (gray spheres in the picture) of zeolites to form M+ ⋅⋅⋅CO and M+ ⋅⋅⋅OC species (C: black; O: white) which are in a temperature-dependent equilibrium. For Na-ZSM-5 (M+ =Na+ ) the difference in interaction energy amounts to 3.8 kJ mol-1 , as determined by means of variable-temperature FT-IR spectroscopy.

Dipole coupling and chemical shifts of CO and NO adsorbed on oxides and halides with rock-salt structure

The infrared spectra of CO and NO adsorbed at 77 K on sintered alkali-metal halide films (LiF, NaCl, KCl and NaI) and on sintered polycrystalline oxides (MgO and NiO) are reported. The most intense bands have been assigned to monomeric (CO) and dimeric (N2O2) species adsorbed on (100) faces. The CO molecules are adsorbed in a localized form on surface cations with orientation perpendicular to the (100) planes and form a regular array of parallel oscillators which interact via dynamic and static effects. The dynamic polarizability (αv) of CO adsorbed on alkali-metal halides and magnesium oxide is very small (αv≈ 0.02–0.03), while that of CO on NiO is one order of magnitude larger. The molecules of CO adsorbed on (100) faces also interact through the solid via inductive effects. NO is adsorbed on (100) planes of alkali-metal halides and MgO in the dimeric cis form; on NiO monomeric nitrosylic species predominate.

Hydrogen storage in Chabazite zeolite frameworks

We have recently highlighted that H-SSZ-13, a highly siliceous zeolite (Si/Al = 11.6) with a chabazitic framework, is the most efficient zeolitic material for hydrogen storage [A. Zecchina, S. Bordiga, J. G. Vitillo, G. Ricchiardi, C. Lamberti, G. Spoto, M. Bjørgen and K. P. Lillerud, J. Am. Chem. Soc., 2005, 127, 6361]. The aim of this new study is thus to clarify both the role played by the acidic strength and by the density of the polarizing centers hosted in the same framework topology in the increase of the adsorptive capabilities of the chabazitic materials towards H2. To achieve this goal, the volumetric experiments of H2 uptake (performed at 77 K) and the transmission IR experiment of H2 adsorption at 15 K have been performed on H-SSZ-13, H-SAPO-34 (the isostructural silico-aluminophosphate material with the same Brønsted site density) and H-CHA (the standard chabazite zeolite: Si/Al = 2.1) materials. We have found that a H2 uptake improvement has been obtained by increasing the acidic strength of the Brønsted sites (moving from H-SAPO-34 to H-SSZ-13). Conversely, the important increase of the Brønsted sites density (moving from H-SSZ-13 to H-CHA) has played a negative role. This unexpected behavior has been explained as follows. The additional Brønsted sites are in mutual interaction via H-bonds inside the small cages of the chabazitic framework and for most of them the energetic cost needed to displace the adjacent OH ligand is higher than the adsorption enthalpy of the OH...H2 adduct. From our work it can be concluded that proton exchanged chabazitic frameworks represent, among zeolites, the most efficient materials for hydrogen storage. We have shown that a proper balance between available space (volume accessible to hydrogen), high contact surface, and specific interaction with strong and isolated polarizing centers are the necessary characteristics requested to design better materials for molecular H2 storage.